ScuzzBlog: Diaries October 2020

Entry 5th October 2020: Post 1: Amiga 500 - 1MB CHIP RAM.

Amiga 500 - 1MB CHIP RAM.

OK Get ready to set your faces to STUN !!

First up I bought these two computers from the same person back in
the early 2000's. The girl that I collected them from said they had
belonged to her grandfather. I sense he was no longer with us. Both
computers had a rear switch which intrigued me during the auction
and was reason for my purchase. The A500Plus has the ROM switcher 
and which I use a lot with a 590 sidecar. The second is a little 
more of a mystery.

When you crack the case you find a Power Computing 1.5MB upgrade
card which I removed the clock battery. There is a small card mod
to the GARY and a wire to JP2 which I assumed is a timing mod to
try and get the slow and fast working at the same speed.

The thing you notice most is the hot glue all over the place. All
the chips have been hot glued and the mess is all over the board. At
first it looks random but I sense some was to hold wires.

The CHIP RAM looks like kinda original and gives 1MB on the board with
the FAT AGNUS at 1MB as this is a REV 6A board. I can't believe it was
fitted with 1MB when it came and so this was upgraded. This upgrade 
normally would involve the cutting of tracks at JP2 and soldering the 
upper two together. The whole board is further complicated by the 
addition of the 1 MB 4 x W-GER TC514256-10R chips to the vacant slots. 
This gives 1MB slow.

*4pc of 1 meg Dip 20 Memory  256 x 4 Toshiba TC514256AP-80 Fast Page 
Dynamic Ram  Fast Page DRAM, 256KX4, 80ns, CMOS, PDIP20

I talk with ignorance cus I haven't found anything on the actual card
or the smaller Power Computing GARY mod card, though I think this is
all done to access the 1MB CHIP and the 1MB SLOW but allowing the
card RAM to be coupled with the main RAM to give more CHIP. Though
don't quote me on it. The four vacant on the original board was always
the slower RAM that wasn't used with the custom chips. Hence the 
SLOW RAM or ... duh FAST RAM. Bonkers. When Fast is on the same BUS
as the CHIP RAM it is slow FAST memory.

Why he felt the need to glue everything down I know not, unless he
was worried that if someone lifted or changed the chips it may cause
an issue. Why ? No idea.

The reason for me taking a further look at the machine is that the
SYSINFO from a couple of year back now said it only had 300K of
CHIP and this made no sense, so I switched her on.

SHE DIDN'T WORK.. Bugga. The floppy drive was halted at boot and no
activity. I pulled the wire connector from the card and I could use
just one disk. Any other disk used would be destroyed, or give a
checksum error rendering it unusable. I got through several disks
and used external drives, giving all the same errors. I formatted
disks on A500 and A1200 but it wasn't any different.

So I pulled the card completely and the machine worked without fault.
SYSINFO gives 1MB CHIP and 1MB SLOW. The main Workbench screen shows
1.9MB available. So the card was not giving anything to the machine.

The switch at the rear is wired to the AUDIO FILTER section though
appears to do nothing. Not that I can tell anyway. I ran a check
on memory but that showed nothing. I couldn't run a test on the
card cus I am unable to get a second floppy disk working and
Workbench is needed for the software.

The card did suffer some corrosion from the battery but not enough
to kill it. I have had way worse.

My thoughts on completing the process is that the 300K on the previous
showing was an error as it was picking something from the card.
I did have the card installed last time but the memory was only
just 1.19MB and so the memory was deficient. Not sure it was really
working off the card. The card is not picked up by SYSINFO.

I am no expert but I sense that the guy knew what he was doing.
The second A500Plus with the ROM switcher probably answered all his
needs and so he probably shelved the A500 in its favour. The extra
goo on  the A500 may have been where he had the ROM switcher or
attempted to but gave up and just used the A500Plus. Albeit that
the A500Plus base motherboard tray is not the original. Its a bit
of a mystery.

I have disabled the board. I think the GARY interface may be still
working to give the 1MB CHIP but the mod to access the SLOW as CHIP
isn't. The card memory may just be fried. I will check in another
computer at some time.

In the end when I cleaned up and pressed down on everything and
reseated the GARY and connected the card back up I finished with
a yellow screen so its not even getting past the memory test. I
did try the MEMTEST but the computer locked up with the card connected
and connector removed.

You know I was at this thing for most part of the day. Fun though.
Saying all that I am none the wiser. I was sure the A500 only ever
came with 512K CHIP.  Don't think I'm done here.

I was tempted to get some 99% Isopropyl Alcohol and remove the
hot glue but given she is working for the most part I decided to 
leave it for now.

PS I am totally lost as to how you identify the RAM capacity of
chips ... and so answers on a postcard please. I believe the RAM 
is FAST PAGE DRAM 256Kx4 but that appears the same on a 512K A500.

Amiga 500 - 1MB CHIP RAM.

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Last updated 6th October 2020

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